Antenna device applicable for two different frequency bands

ABSTRACT

A branching filter using dielectric elements is disposed on the side of one of a pair of reflectors for reflecting emitted or incident waves, the branching filter being capable of reflecting one of the two frequency group components and of transmitting the other frequency group component to branch and converge the two components at different points.

343-9 1 1 R AU 0 Umted States Patent 1 1 1 1 3,763,493 Shimada et al.Oct. 2, 1973 ANTENNA DEVICE APPLICABLE FOR TWO [56] References CitedDIFFERENT FREQUENCY BANDS UNITED STATES PATENTS 1 lnvemrs= Shim,Kosanei-shi, 3,281,350 l0/l966 Hannan 343 9119 T y Maslki y 3,394,3787/l968 Williams et al...... Sayama-shi; Hiroyukl Kumazawa, 2,972,7432/1961 Svensson et al. 343/909 Saitama-ken, Tokorozawa-shi; Mmhimxmkomi, suginambku, FOREIGN PATENTS 0R APPLICATIONS Tokyo, all of Japan335,425 2/1959 Switzerland 343/837 562,602 9/1958 Canada 343/756 [73]Assigneez Nippon Telegraph and Telephone Pubhc Corponuon Tokyo JapanPrimary Examiner-Eli Lieberman [22] Filed: Oct. 6, 1971 Attorney-MiltonJ. Wayne Appl. N0.; 186,911

Foreign Application Priority Data Oct. 17, 1970 Japan 45/91422 US. Cl343/755, 343/781, 343/837, 343/911 Int. Cl. H0lq 19/14 Field of Search343/840, 909, 755,

[57] ABSTRACT A branching filter using dielectric elements is disposedon the side of one of a pair of reflectors for reflecting emitted orincident waves, the branching filter being capable of reflecting one ofthe two frequency group components and of transmitting the otherfrequency group component to branch and converge the two components atdifferent points.

7 Claims, 13 Drawing Figures Patented Oct. 2, 1973 3,763.493

4 Sheets-Sheet 1 PRIORART 2 PRIORART FIG. 5 (b) 4 Sheets-Sheet 2 FIG. 6

-- FREQUENCY (GHZ) Patented Oct. 2, 1973 O O O 4 Patented Oct. 2, 1973 4Sheets-Sheet 3 FIG. 8

FIG. 9

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Patented Oct. 2, 1973 3,763,493

4 Sheets-Sheet 4 FIG.||

ANTENNA DEVICE APPLICABLE FOR TWO DIFFERENT FREQUENCY BANDS BACKGROUNDOF THE DISCLOSURE The present invention relates to an antenna devicecomprising a pair of reflectors and more particularly an antenna devicein which a branching filter using dielectric elements is disposedadjacent to one of a pair of reflectors so that the waves of twodifferent frequency groups may be directly branched or composed in theantenna section.

In the telecommunication systems utilizing the telecommunicationsatellites as the relay stations, there has been proposed the use of twoor more frequency bands such as the microwave frequency band of 4 6GI-Iz and the quasi-millimeter frequency band of i7 30 GHz. The antennadevices used in such telecommunication system must have the capabilityof simultaneously composing or branching the frequency groups in themicrowave and quasi-millimeter frequency bands. This is accomplished byuse of the frequency group branching filters connected to the Cassegrainantennas which are widely used in the microwave communication systems.In antenna devices of the type described, two wave guides are coupledthrough many slots that are equidistantly spaced apart to constitute awave-guide type frequency group branching filter which in turn iscoupled to the feeder section of the antenna device. In case oftransmission the frequency group branching filter composes the twosignal components of the microwave and quasi-millimeter frequency bandswhich are transmitted by the two different wave guides respectively, andthe composed signals are transmitted to a primary horn through a feedersection, reflected by a subreflector and a main reflector and emittedinto the space. In case of reception, the incident waves are reflectedfirst by the main reflector and then by the subreflector to betransmitted into the feeder section through the primary horn, and arebranched into the microwave and millimeter frequency group signalcomponents by the frequency group branching filter to be fed into thetwo different wave guides respectively.

However, the number of slots in the frequency group branching filtermust be increased as the frequency of the signals transmitted isincreased. Furthermore since the signals of the quasi-millimeterfrequency band which is four to five times higher than the microwavefrequency band are transmitted or received simultaneously with thesignals of the microwave frequency band, the feeder section coupled tothe primary horn of the antenna device tends to become oversizedrelative to the signals in the quasi-millimeter band. Therefore theundesired high order mode of the quasimillimeter band tends to beexcited in the frequency group branching filter and the feeder sectionand so the quality of the transmitted signals is much deteriorated. Theundersired high order mode produced causes tracking error when theantenna device is steered to track a communication satellite.Furthermore, there is heat loss in the walls of the wave guides of thefrequency group branching filter when the signal current flows. Thisheat loss is a cause of the deterioration of the gain-noise temperatureratio (6/1) of the antenna device. Therefore it is not preferable to usea frequency group branching filter of the type comprising wave guides inorder to branch or compose the wide band signal waves extending from arelatively lower frequency group to a relatively higher frequency groupwhich is several times higher than the lower frequency group infrequency.

One of the objects of the present invention is therefore to provide anantenna device in which the frequency group branching and compositionmay be carried out directly in the antenna section without using thewave-guide type frequency group filter.

Another object of the present invention is to provide .an antenna deviceincorporating a branching filter using dielectric elements which iscapable of reflecting a high frequency beam of the two differentfrequency group components and of transmitting therethrough the otherfrequency group component so that the frequency group branching andcomposition of the beam may be effected.

According to one embodiment of the present invention, first and secondreflectors are disposed in opposing relation, and a third reflector or abranching filter using dielectric elements capable of reflecting thewaves in the higher frequency group and of transmitting therethrough thewaves in the lower frequency group is disposed adjacent to one of thefirst and second reflectors, for example the second reflectorqlnreception the incident waves in the low frequency group are firstreflected by the first reflector, transmitted through the thirdreflector and reflected again by the second reflector to converge towarda primary horn for low frequency group. The waves in the high frequencygroup are first reflected by the first reflector and then by the thirdreflector or branching filter to converge toward a primary horn for highfrequency group. In case of transmission the direction of thepropagation or transmission of the waves in the high and low frequencygroups are reversed.

In the antenna device of the present invention, at least one of thefirst, second and third reflectors is a paraboloidal reflector while theother two reflectors are hyperboloidal reflectors. Alternatively one ofthe three reflectors is a plane reflector which is disposed at an anglerelative to the axes of the paraboloidal or hyperboloidal reflectors.The former type antenna device in accordance with the present inventionis especially adapted for use in an earth station in the satellitetelecommunication system using two or more frequency bands, while thelatter type antenna device is adapted to be mounted on atelecommunication satellite.

BRIEF DESCRIPTION OF THE DRAWINGS FIGJ is a schematic sectional view ofa prior art Cassegrain antenna device incorporating therein a waveguidetype frequency group filter;

FIG .2 is a fragmentary longitudinal sectional view, on enlarged scale,of the frequency group filter thereof;

FIG.3 is a sectional view taken along the line 3-3 of FIGJ;

FIG.4 is a schematic sectional view illustrating one embodiment of anantenna device in accordance with the present invention;

FIGS-(a) is a perspective view of a branching filter using dielectricelements used in the antenna device in accordance with the presentinvention;

FIGS-(b) is a diagram for explanation of the principle of operationthereof;

FIG.6 is a graph illustrating the frequency characteristic curves of thebranching filter shown in FIGS;

FlGS.7-9 are schematic sectional views of some variations of the antennadevice in accordance with the present invention;

FlG.10 is a sectional view of the antenna device of the presentinvention used as an antenna for an earth station;

FIG.11 is a schematic sectional view illustrating the antenna device inaccordance with the present invention used as a satellite antenna;

FlG.12 is a view for explanation of the mode of operation thereof; and

FIG. 13 is a cross sectional view of a reflector which may be employedin the antenna device of FIG. 4.

DESCRIPTION OF THE PREFERRED EMBODIMENTS Prior Art Referring to FlG.1, aprior art Cassegrain antenna device generally designated by 10 comprisesa main reflector 11, a subreflector 12, a primary horn 13 located at thecenter of the main reflector 11, a feeder section 14, and a frequencygroup branching filter including a pair of wave guides 16 and 17. Incase of transmission, two signal components of different frequency bandssuch as quasi-millimeter frequency band and the microwave frequency bandwhich are respectively transmitted through the wave guides 16 and 17 arecomposed by the frequency group branching filter 15 and led to feedersection 14 and transmitted from the primary horn 13. The electromagneticwaves emitted from the primary horn 13 are reflected by the subreflector12 and the main reflector 11 in the order named and transmitted intospace. In case of reception, the incident waves are reflected by themain reflector 11 to the subreflector 12 from which they are led intothe primary horn 13, the feeder section 14 and the frequency groupbranching filter 15 where they are branched into the signal componentsof the quasi-millimeter frequency band and of the microwave frequencyband respectively. The branched signals are transmitted through the waveguides 16 and 17 respectively to a reception apparatus (not shown).

Next referring to FlGS.2 and 3 illustrating the enlarged sectional viewsof the frequency group branching filter 15, the two wave guides 16 and17 are coupled to each other through a plurality of slots 18 which aresubstantially equidistantly spaced apart from each other, and thetransmitted wave frequency band characteristic of the coupler is soselected as to comply with the quasi-millimeter frequency band.Therefore in case of the reception, the incident waves fed from thefeeder section 14 in the direction indicated by the arrow 19 into thefrequency band branching filter 15 are so branched that the signalcomponents in the quasimillimeter frequency band are transmitted throughthe slots 18 into the wave guide 16 while the signal components in themicrowave frequency band are transmitted through the wave guide 17. Incase of transmission, the propagation of these signal components is ofcourse reversed, and the signal components in the quasimillimeter andmicrowave frequency bands which are composed by the group frequencybranching filter 15 are led toward the primary feeder 14.

However, as stated previously, it is not preferable to couple thefrequency group branching filter of the type described comprising a pairof wave guides 16 and 17 to the feeder section of the Cassegrain antennain order to branch and compose the waves widely extending in frequencyfrom the microwave frequency to the quasimillimeter frequency band,because of many associated problems.

THE INVENTION FlG.4 is a schematic sectional view of an antenna devicein accordance with the present invention for explanation of theunderlying principle thereof. A main reflector 101 is a paraboloidalreflector with the focal point indicated by 102, and a firstsubreflector 103 that is a paraboloidal reflector with the focal pointindicated by 102 and a second subreflector 104 which is a hyperboloidalreflector are disposed in opposing relation with the main reflector 101.The second subreflector 104 comprises a branching filter usingdielectric elements to be described in more detail hereinafter, so thatthe electromagnetic waves in higher frequency bands such as thequasi-millimeter frequency band may be reflected while the waves in thelower frequency bands such as the microwave frequency band may betransmitted therethrough. A primary horn 106 for high frequency band islocated at the conjugate focal point 105 of the focal point 102 of thehyperboloidal subreflector 104, and another primary horn reflector 107for lower frequency band is located at the center of the main reflector101.

Now it is assumed that the waves in the low frequency band indicated bythe solid lines 108 and the waves in the higher frequency band indicatedby the dashed lines 109 arrive at the same time at the antenna device100. The waves 108 in the lower frequency band are reflected by the mainreflector 101, transmitted through the second subreflector 104 or thebranching filter using the dielectric elements and then reflected againby the subreflector 103 to be made incident onto the primary horn 107 asplane waves. The incident waves are converged at the focal point 110 ofthe primary horn reflector 107 and derived in the direction indicated bythe arrow 111. The waves in the higher frequency band 109 are firstreflected by the main reflector 101 and then by the second hyperboloidalsubreflector 104 or branching filter using dielectric elements,converged at the focal point 105 and made incident onto the primary horn106, from which the waves are derived in the direction indicated by thearrow 112. It is of course understood readily that in case oftransmission the propagation paths of the waves are reversed.

As described above according to the present invention the waves in thehigher and lower frequency bands are branched or composed directly atthe antenna section so that oversized wave guides coupled to the primaryhorns 106 and 107 are not required. Furthermore the branching andcomposition of the waves over the wide frequency band may be effectedwith a negligible loss and the minimum of undesired higher order mode.

FIG .5 illustrates a branching filter using dielectric elements forbranching two waves in the two frequency bands. For the simplicity ofexplanation, the frequency bands used are the quasi-millimeter frequencybands of 18 GHz and 26 GHz and the microwave frequency bands of4 GHz and6 GHz. Referring to FIGS-(a), the branching filter 200 is shown ascomprising a plurality of sheet-shaped dielectric elements 201-205 oftwo types having different dielectric constants. The thickness of theelements 201-205 is substantially equal to,-

for example, one fourth of the wave length of the center frequency (23.56112), of the two quasi-millimeter frequency bands of 18 and 26 61-12.The filter 200 is inclined as shown in FlG.5-(b), and the composed waves206 in the above stated quasi-millimeter and microwave frequency bandsare made incident to the filter 200. The substantial components in thequasimillimeter frequency bands are reflected by the first dielectricelement 201 as indicated by 211, and the remaining components aretransmitted through the element 201 as indicated by 207. The transmittedwaves 207 are then reflected by the second dielectric element 202 sothat the substantial components in the quasimillimeter frequency bandsincluded in the transmitted waves 207 are reflected as shown by 212. Theremaining components are transmitted through the second dielectricelement 202 as indicated by 208. In a manner similar to that describedthe waves are successively reflected by and transmitted through thesuccessive dielectric elements 203-205. As a consequence the substantialcomponents in the quasi-millimeter frequency bands are derived as thereflected waves 211, 212 and 213 while the substantial components in themicrowave frequency bands are derived as the transmitted waves 210. FIG.13 is a cross sectional view of the combination of the firstsubreflector 103 and second subreflector 104 of FIG. 4 employing thebranching filter as illustrated in FIG. 5(b).

The frequency characteristic curves of the filter 200 shown in F165 areillustrated in FlG.6. The solid curves indicate the theoretical valueswhile the dotted line curves, those marked with measured," indicate themeasured values. The frequency is plotted against the abscissa while thetransmission loss (T) and reflection loss (R) against the ordinate. FromFIG.6, it is seen that the filter shown in FIGS has the excellentcapability of branching the waves in the quasi-millimeter frequency bandof 18-26 61-12 from those in the microwave frequency band of 4-6 61-12.

The present invention is not limited to the arrangement shown in FlG.4,and various variations and modifications can be effected as will bedescribed hereinafter by reference to FlGS.7-9 without departing fromthe scope of the invention.

In a variation illustrated in FlG.7, both of the first and secondsubreflectors for reflecting the waves in the lower and higher frequencybands are hyperboloidal reflectors at one focus or focal points of whichare located the primary horns respectively. More specifically, one ofthe focal points of the first hyperboloidal subreflector 303 is locatedto be coincident with the focal point 302 of the paraboloidal mainreflector 301. The other focal point or focus of the hyperboloidalsubreflector 303 is indicated by 308. The axis connecting the points orfocal points 302 and 308 of course makes an angle with respect to thatof the paraboloidal main reflector 301. The primary horn 310 forreception of the lower frequency bands is located at the focal point 308of the first subreflector 303 in opposed relation therewith. One focalpoint of the second hyperboloidal subreflector or branching filter 304similar in construction to the filter shown in FIGS coincides with thefocal point of the main reflector 301. The other focal point isindicated by 309. The axis of the second hyperboloidal subreflector 304is also inclined at an angle relative to the axis of the main reflector301, but on the opposite side relative to the axis of the firstsubreflector 303. The primary horn 311 for reception or transmission ofthe higher frequency band is located at the focal point 309 of thesecond subreflector 304 in opposed relation therewith and is alignedwith the axis thereof.

The incident waves in the lower frequency band indicated by the solidlines 314 are first reflected by the main reflector 301 to convergetoward the focal point 302 and then transmitted through the secondsubreflector or branching filter 304 and again reflected by the firstsubreflector 303 to converge toward the focal point 308. Consequentlythe waves in the lower frequency band are made incident into the primaryhorn 310 and derived in the direction indicated by the arrow 313. Theincident waves in the higher frequency band indicated by the dashedlines 315 are first reflected by the main reflector 301 and then by thesecond subreflector or filter 304 to converge toward the focal point 309at which the primary horn 311 is located so that the waves may bederived in the direction indicated by the arrow 314.

An antenna device generally designated by 400 in FIG.8 comprises a planereflector 401 which is disposed at about 45 relative to the axes of afirst and a second paraboloidal reflectors 402 and 403. The secondparaboloidal reflector 403 comprises a branching filter using dielectricelements. The incident waves in the lower frequency band indicated by410 are first reflected by the plane reflector 401, transmitted throughthe second paraboloidal reflector 403 and then reflected again by thefirst paraboloidal reflector 402 to converge toward the focal point 404,at which is located a primary horn 408 so that the waves in the lowerfrequency band may be derived in the direction indicated by the arrow406. The incident waves 411 in the higher frequency band are firstreflected by the plane reflector 401 and then by the second paraboloidalreflector 403 to converge toward the focal point 405, at which islocated a primary horn 409 so that the incident waves may be derived ithe direction indicated by the arrow 407.

A still further variation of an antenna device in accordance with thepresent invention illustrated in FIG .9 is similar in construction tothat shown in FlG.8 except that a branching filter 502 is flat and isdisposed on the side of a first plane reflector 501 in spaced apartrelation therewith. A paraboloidal reflector 503 is disposed in opposedrelation with the first plane reflector 501 and the second planereflector or the branching filter 502. As in the case of the antennadevices described hereinbefore, the antenna device 400 is provided witha primary horn 504 for reception and transmission of the lower frequencyband and another primary horn 505 for the reception and transmission ofthe higher frequency band. The waves in the lower frequency band arepropagated as indicated by 506 while the waves in the higher frequencyband are propagated as indicated by 507. The mode of operation issimilar to that of the antenna device shown in FlG.8 so that nodescription will be made.

As described above, the antenna devices of the present invention mayhave various arrangements, and those shown in FlGS.4 and 7 areespecially adapted for use in the earth stations of the satellitecommunication systems, while those shown in FIGS.8 and 9 are adapted foruse as the satellite antennas.

An antenna for an earth station in accordance with the present inventionis schematically illustrated in FlG.10. The antenna device comprises amain paraboloidal reflector 601 with a focal point indicated by 623, afirst paraboloidal subreflector 602 made of metal with its focal pointindicated by 623 and a second hyperboloidal subreflector 603 or filterof the type described comprising a plurality of dielectric elements. Oneof the foci or focal points of the second subreflector 603 coincideswith that of the first subreflector 602. The first and secondsubreflectors 602 and 603 are supported by stays 604. The antenna deviceis provided with a horn reflector 605 for reception of the lowerfrequency band and a primary horn 624 for reception of the higherfrequency band which is located at the other focus 606 of thehyderboloidal reflector 603. Lower and higher frequency band receptionapparatus 607 and 608 are coupled through rotary joints 609 and 610 tothe primary horns 605 and 624 respectively. Since the antenna device iselevated about the axis 616, the transmission and reception apparatus607 and 608 which are fixed, are coupled to the primary horns 605 and624 through the rotary joints 609 and 610 coaxial with the axis 616.

When the waves in the higher and lower frequency groups arrivesimultaneously, the waves in the lower frequency group are reflected bythe main reflector 601, transmitted through the second subreflector 603and reflected by the first subreflector 602 to form the plane waveswhich are converged by the horn reflector 605 and transmitted to thetransmission and reception apparatus 607. The waves in the higherfrequency group are reflected by the main reflector 601 and then by thesecond hyperboloidal reflector or dielectric filter 603 to convergetoward the focal point 606 at which is located the primary horn 624. Thewaves are then transmitted to the transmission and reception apparatus608. In case of transmission, the wave propagation paths are reversed.

The main reflector 601 is supported by the stays 617 and 618 securelyfixed to the main reflector and an altitude-azimuth mount 619respectively, and is adapted to rotate about the altitude axis 616 asdescribed previously by a motor 614 through a gear 613 which is carriedby a motor drive shaft 615 and is in mesh with a gear 612 carried by thealtitude shaft 616. The motor 614 is securely fixed to the mount 619which in turn rides on a rail 620 on a foundation 621 to rotate aboutthe axis 622. Therefore the elevation or altitude and azimuth of theantenna device may be selected to track a communication satellite.

FlG.11 is a schematic view illustrating a mechanical despun antenna of asatellite. In general, the geostationary satellite is spun in order tostablize its hovering and then the satellite antenna must be rotated inthe di rection opposite to the direction of spin of the satellite inorder to direct the antenna beam toward the earth. in case of thesatellite antenna in accordance with the present invention which is notsymmetrical about the axis of spin, both of the primary horns andtransponders must be rotated as the antenna device is rotated. Theantenna device of this type is called a platform despun antenna.

The satellite antenna illustrated in FIG.11 comprises a metalparaboloidal reflector 701 with an axis 703 and a focus or focal point706, another paraboloidal reflector 702 comprising a filter usingdielectric elements with an axis'704 and a focus or focal point 705, alower frequency group primary horn 708 located at the focal point 706, ahigher frequency group primary horn 707 located at the focus or focalpoint 705, a plane reflector 709 disposed at about 45 relative to theaxes 703 and 704 and supported by stays 711 and an arm 710 or supportingthe reflectors 701 and 702. The satellite 712 carries a solar cell 713,a despun motor 714 whose rotary shaft 715 is securely fixed to thereflectors 701 and 702, the primary horns 707 and 708 and a platform718, high and low frequency group transponders 716 and 717 and an apogeemotor 719.

In case of reception, the waves in the lower frequency group arereflected by the plane reflector 709, transmitted through the reflector702 and reflected again by the reflector 701 to converge toward thefocal point 706. The converged waves are fed to the transponder 717through the primary horn 708 which is located at the focal point 706. Inlike manner waves in the higher frequency group are reflected first bythe plane reflector 709 and then by the reflector 702 to converge towardthe primary horn 707 which is located at the focal point 705, and aretransmitted toward the transponder 716.

The rotation of the antenna device is effected by the stator 714 fixedto the satellite proper and the rotary shaft 715 fixed to the platform718 and the primary horns of the antenna device, the stator and therotary shaft constituting a despun motor. Therefore, the antennaelements, the primary horns and the transponders may rotate in unison inthe direction opposite to the direction of spin of the satellite properso that the beam may be always directed toward the earth.

The following advantages may be accrued from the satellite antenna shownin FIG. First there is no blocking because the primary horns and theirstays are not disposed in the propagation paths. Secondly the reflectorshaping of the two reflectors may be effected relative to each other sothat the high antenna efficiency and the easy beam shaping may beattained.

FlG.12 schematically illustrates a geostationary satellite 804 having anantenna device comprising a reflector assembly 801 including a pair ofparaboloidal reflectors one of which is a dielectric filter of the typedescribed, and a plane reflector 802 which is disposed at an anglerelative to the axis 803 of the reflector assembly 801. Thegeostationary satellite 804 spins about the axis 803 which isperpendicular to a plane including the equator of the earth 805 so thatwhen the plane reflector 802 is inclined at 45 relative to the axis 803,the antenna beam is directed toward the equator as indicated by 806. Inorder to direct the beam to, for example, Japan as indicated by 806',the plane reflector 802 is inclined as indicated by the dashed line802'. In such a simple manner as described above, the beam may bedirected to any desired direction without adversely affecting theantenna characteristics.

Although the dielectric filter has been described as being capable oftransmitting the waves in the lower frequency group and of reflectingthe waves in the higher frequency group, it may be designed to reflectthe waves in the lower frequency group and to transmit the waves in thehigher frequency group.

What is claimed is:

1. An antenna for reflecting emitted and incident wave components offirst and second frequency groups, comprising first and secondreflectors positioned in opposed relation to sequentially reflect wavecomponents, and a third reflector positioned adjacent one of said firstand second reflectors, said third reflector being comprised of abranching filter having a plurality of dielectric layers positioned toreflect wave components of said second group and transmit wavecomponents of said first group, said first and second reflectors beingpositioned to converge wave components of said first group at a firstpoint, and said third and otherof said first and second reflectors beingpositioned to converge wave components of said second group at a secondposition spaced from said first position, said branching filter beingcomprised of a plurality of alternate layers of materials of twodifferent dielectric constants with thickness substantially equal toone-quarter of the wave length of the center frequency of the secondgroup.

2. The antenna of claim 1 wherein said one reflector is said secondreflector, said first and second reflectors are paraboloidal with acommon focal point, and said third reflector is positioned between saidfirst and second reflectors and is hyperboloida] with a focus at saidcommon point.

3. The antenna of claim 1 further comprising a horn for said secondgroup at the conjugate focal point of said common focal point, and asecond horn for said group positioned at the center of said firstreflector.

4. The antenna of claim 3 wherein said first group is in the microwavefrequency band and said second group is in the quasimillimeter band.

5. The antenna of claim 1 wherein said one reflector is said secondreflector, said second and third reflectors are hyperboloidal, saidfirst reflector is paraboloidal with a focus common with one focus ofsaid second reflector a horn at the other focus of said second reflectorwith the axis of the two focal points of the second reflector at anangle to the axis of said first reflector, one focus of the thirdreflector coinciding with the one focus point of said second reflector,and a horn at the other focus point of said third reflector, the axis ofthe focal points of said third reflector being at a different angle tothe axis of the first reflector.

6. The antenna of claim 1 wherein said one reflector is said firstreflector, said first and third reflectors are paraboloidal, and saidsecond reflector is a plane reflector at an angle to the axis of thefirst reflector, the axes of said first and third reflectors beingspaced apart.

7. The antenna of claim 1 wherein said one reflector is said secondreflector, said first reflector is paraboloidal and said second andthird reflectors are plane reflectors at different angles to the axis ofsaid first reflectOT.

##tit

1. An antenna for reflecting emitted and incident wave components offirst and second frequency groups, comprising first and secondreflectors positioned in opposed relation to sequentially reflect wavecomponents, and a third reflector positioned adjacent one of said firstand second reflectors, said third reflector being comprised of abranching filter having a plurality of dielectric layers positioned toreflect wave components of said second group and transmit wavecomponents of said first group, said first and second reflectors beingpositioned to converge wave components of said first group at a firstpoint, and said third and otherof said first and second reflectors beingpositioned to converge wave components of said second group at a secondposition spaced from said first position, said branching filter beingcomprised of a plurality of alternate layers of materials of twodifferent dielectric constants with thickness substantially equal toone-quarter of the wave length of the center frequency of the secondgroup.
 2. The antenna of claim 1 wherein said one reflector is saidsecond reflector, said first and second reflectors are paraboloidal witha common focal point, and said third reflector is positioned betweensaid first and second reflectors and is hyperboloidal with a focus atsaid common point.
 3. The antenna of claim 1 further comprising a hornfor said second group at the conjugate focal point of said common focalpoint, and a second horn for said group positioned at the center of saidfirst reflector.
 4. The antenna of claim 3 wherein said first group isin the microwave frequency band and said second group is in thequasimillimeter band.
 5. The antenna of claim 1 wherein said onereflector is said second reflector, said second and third reflectors arehyperboloidal, said first reflector is paraboloidal with a focus commonwith one focus of said second reflector a horn at the other focus ofsaid second reflector with the axis of the two focal points of thesecond reflector at an angle to the axis of said first reflector, onefocus of the third reflector coinciding with the one focus point of saidsecond reflector, and a horn at the other focus point of said thirdreflector, the axis of the focal points of said third reflector being ata different angle to the axis of the first reflector.
 6. The antenna ofclaim 1 wherein said one reflector is said first reflector, said firstand third reflectors are paraboloidal, and said second reflector is aplane reflector at an angle to the axis of the first reflector, the axesof said first and third reflectors being spaced apart.
 7. The antenna ofclaim 1 wherein said one reflector is said second reflector, said firstreflector is paraboloidal and said second and third reflectors are planereflectors at different angles to the axis of said first reflector.